1. Field of the Invention
The present invention relates to wireless communications and, more particularly, to wireless signal repeaters. Such repeaters can be usefully employed to enhance wireless communications of various types. For purposes of example, this description will focus mainly on cellular wireless communications, such as CDMA communications for instance.
2. Description of Related Art
Cellular wireless is an increasingly popular means of personal communication in the modern world. People are using cellular wireless networks for the exchange of voice and data over cellular telephones, personal digital assistants (PDAs), cellular telephone modems, and other devices. In principle, a user can seek information over the Internet or call anyone over a public switched telephone network (PSTN) from any place inside the coverage area of the cellular wireless network.
In a typical cellular wireless system, an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective base station antenna. The base station antennae in the cells are in turn coupled to a base station controller, which is then coupled to a telecommunications switch (e.g., mobile switching center) or other gateway (e.g., packet data serving node) that provides connectivity with a transport network such as the PSTN or the Internet.
When a mobile station (such as a cellular telephone, pager, or appropriately equipped portable computer, for instance) is positioned in a cell, the mobile station communicates via an RF air interface with the BS antenna of the cell. This air interface communication can occur over control and traffic channels as defined by an accepted protocol. Communications from the base station to a mobile station are considered to be in a “forward” direction, so the air interface channels used to carry such communications are referred to as the “forward link” channels. Conversely, communications from a mobile station to the base station are considered to be in a “reverse” direction, so the air interface channels used to carry such communications are referred to as “reverse link” channels.
For instance, in a Code Division Multiple Access (CDMA) system operating according to industry standard IS-2000 (now commonly known as “cdma2000”), up to 64 communication channels exist on the forward link, each distinguished by a unique “Walsh code.” The control channels include a pilot channel defined by Walsh code 0, a synch channel defined by Walsh code 32, and a number of paging channels defined by Walsh codes 1 through 7, as necessary. The traffic channels, in turn, are defined by the remaining Walsh codes (up to 62 in total).
Further, in a CDMA system, each physical sector in a cell is distinguished by a PN offset, which defines a sector-specific part of a pseudo-random number. Communications between a mobile station and the base station on a given channel, in a given sector, and on a given carrier frequency, are encoded using the Walsh code of the channel and the PN offset of the sector and are then carried on the carrier frequency. A receiving entity can then extract particular channels from the air interface by employing a “rake receiver” that scans through air interface signals in search of signals that are encoded with particular combinations of PN offset and Walsh code.
Under cdma2000, the pilot channel is used for establishing signal timing and conveying signal strength measurements to facilitate handoff between sectors. The pilot channel does not carry messages but rather provides a stream that mobile stations can frequently sample to determine how well they can hear a base station. A mobile station will measure the pilot when it initializes and regularly while it's powered up, whether idle or in a call, in search of the strongest sector.
The synch channel, on the other hand, is used to convey system identification and other system-specific information to mobile stations, generally to allow mobile stations to establish communication in the sector. For this reason, the synch channel carries one message, the synch channel message, which contains information such as the system time, code and frame timing, an indication of the PN offset for the sector sending the message, and a system ID number. When a mobile station first powers-on in a CDMA system, it will search for the strongest pilot and then check the synch channel to find out what PN offset it is using to communicate with the base station.
A paging channel, in turn, may be used to page a mobile station in order to determine whether the mobile station is available to receive a call. Further the paging channel(s) may carry system information and call setup orders to facilitate establishment of calls with the mobile station. For instance, the base station may send a general page message over a paging channel, which can direct a mobile station to switch to a designated traffic channel and to alert a user of an incoming call. Once a mobile station finds a pilot and checks the synch channel, the mobile will quickly switch to the paging channel to be able to receive calls.
With the continued growth in popularity of wireless communications, people are increasingly engaging in cellular wireless communications in place of traditional landline communications. This is of course most notable within buildings, such as homes and offices, where landline communications, such as traditional telephone and computer networks, were once the only method available. Use of mobile stations instead of fixed landline terminals conveniently allows for free movement within such buildings as well as into and out of the buildings.
Unfortunately, however, cellular wireless communications can suffer from varying levels of signal degradation as signals are carried over the air interface between the mobile station and the base station. Most significantly, for instance, the strength of signals transmitted in the forward link direction from the base station to the mobile station can diminish substantially as the signals travel over increased distance to the mobile station. Further, obstructions such as buildings and foliage can attenuate or otherwise degrade signals passing over the air interface.
When a mobile station operates within a building, the problem can be even more acute, as the structure of the building can further diminish the signal. In a worst case scenario, forward link signals that ultimately arrive at a mobile station within a building may be too weak to be detected and/or used by the mobile station. As a result, service could be unavailable and wireless communications could be blocked or dropped. This is clearly a problem for cellular wireless carriers who are seeking to promote in-building use of their service.
One approach that can be used to overcome this difficulty is to increase the power transmitted by the base station. A disadvantage with this approach, however, is that the increased power can lead to increased interference in adjacent cells. This, in turn, can decrease the capacity of the wireless network. The power increase can also interfere with other wireless devices in use around the base station. The magnitude of the power increase proportionally affects the amount of interference produced by the base station, with a greater power increase causing a greater interference.
Also, this approach would only solve half of the problem. Increasing the signal strength from the base station would allow a mobile unit to receive the base station's signal in areas that highly attenuate that signal. It would not aid the base station in receiving a signal from a mobile station. Cellular service is primarily a bi-directional mode of communication, in which connections are established and maintained through control signals that are sent both directions between the base station and a mobile station. Therefore, in order to support bi-directional communication, an increase in the transmission power of the base station would usually require a corresponding increase in the transmission power of the mobile station.
Another approach to help increase wireless signal strength within a building is to provide a wireless signal repeater to receive, boost and re-transmit communication signals that are being sent between a base station and a mobile station (or, for that matter, between any sort of wireless access point and a mobile station). In this regard, the wireless signal repeater can operate simultaneously to boost signals on both the forward link and the reverse link. Conveniently, the wireless signal repeater can be positioned within a building so as to boost signals for a mobile station operating within the building.